Pile Design and Construction Practice, Fifth edition

4 General principles and practices
(a) (b)
Soil progressively
increasing in
stiffness or
relative density
with increasing
depth
Figure 1.2 Types of bearing pile (a) Friction pile (b) End-bearing pile.
construction, yet the ability of a pile to carry its load is judged on its behaviour under a
comparatively rapid loading test made only a few days after installation. Because of the
importance of such time effects both in fine- and coarse-grained soils the only practicable
way of determining the load-carrying capacity of a piled foundation is to confirm the design
calculations by short-term tests on isolated single piles, and then to allow in the safety factor
for any reduction in the carrying capacity with time. The effects of grouping piles can be
taken into account by considering the pile group to act as a block foundation, as described
in Chapter 5.
1.4 Dynamic piling formulae
Soft highly
compressible soil
Rock or hard
relatively incompressible
soil
The soil mechanics approach to calculating allowable working loads on piles is that of
determining the resistance of static loads applied at the test-loading stage or during the
working life of the structure. Methods of calculation based on the measurement of the resistance
encountered when driving a pile were briefly mentioned in the context of history.
Historically all piles were installed by driving them with a simple falling ram or drop
hammer. Since there is a relationship between the downward movement of a pile under a
blow of given energy and its ultimate resistance to static loading, when all piles were driven
by a falling ram a considerable body of experience was built up and simple empirical
formulae established from which the ultimate resistance of the pile could be calculated from
the ‘set’ of the pile due to each hammer blow at the final stages of driving. However, there
are many drawbacks to the use of these formulae with modern pile-driving equipment particularly
when used in conjunction with diesel hammers. The energy of blow delivered to the
pile by these types increases as the resistance of the ground increases. The energy can also
vary with the mechanical condition of the hammer and its operating temperature. Simple
dynamic formulae are now largely discredited as a means of predicting the resistance of
piles to static loading unless the driving tests are performed on piles instrumented to measure
the energy transferred to the pile head. If this is done the dynamic analyser (see Section 7.3)
provides the actual rather than the assumed energy of blow enabling the dynamic formula to
be used as a means of site control when driving the working piles. Dynamic pile formulae